Television system



Dec. 3, 1940. A. v. BEDFORD TELEVISION SYSTEM Filed July 30, 1938 5Sheets-Sheet l Dec. 3, 1940. A. v. BEDFORD TELEVISION SYSTEM Filed July50, 1938 5 Sheets-Sheet 2 :inventor Gttorneg Dec. 3, 1940. i A. v.BEDFORD 2,223,812

TELEVISION SYSTEM Filed July 30, 1938 5 Sheets-Sheet 5 l Dec.3, 1940. AV, BEDFORD Y 2,223,812

TELEVI S ION SYSTEM Filed July 50, 1938 5 Sheets-Sheet 4 Dec. 3, 1940.A, v BEDFORD 2,223,812

TELEVI S ION SYSTEM Filed July 30, 1938 5 Sheets-SheerI 5` i e sg e. e,S Summer 51 70/1 N99 ya Kvfd Gnome" scanning.

Patented Dec. 3, 1940 UNITED STATES TELEVISION SYSTEM Alda V. Bedford,Collingswood, N. J., assignor to Radio Corporation of America, acorporation of Delaware Application .my at, 193s, sensi No. mausiv i eciiims.A (c1. 17a- 7.2)

My invention relates to television systems and particularly to a methodof and means for improving the synchronization of the scanning at thereceiver with that at the transmitter where interlaced scanning isemployed.

In my Patent No. 2,192,121, issued Feb. Z7, 194), entitled Televisionsystems and the method of operation thereof, and assigned to the RadioCorporation of America, there is described a television system of thetype in which picture signals and horizontal and vertical synchronizingimpulses are transmitted in the same channel, the synchronizing impulseshaving rthe proper frequency relation to produce interlaced scanning,and wherein there 'are impulses occurring in the region of the verticalVsynchronizing impulse at a multiple of the frequency of the horizontalsynchronizing impulses, specifically at double this frequency for doubleinterlacing. Such multiple frequency impulses are desirable, asexplained in said patent, because succeeding vertical synchronizingimpulses occury in different time relations to the horizontalsynchronizing impulses in a system of this type employing odd-lineinterlaced By odd-line interlaced scanning is meant scanning where thevertical deflecting frequency goes into the horizontal deflectingfrequency a whole number of times plus a fraction such as one-half,one-third, one-fourth, or the like. This diiference in time relationofthe impulses tends to cause succeeding vertical synchronizing impulsesas impressed upon the receivers vertical deiiecting circuit to beunlike, whereby the quality of the -interlacing at the receiver isimpaired. However, by employing the multiple frequency impulses, thesucceeding vertical synchronizing impulses are caused to be almostidentical at the receiver defiecting circuit, whereby good interlacingis obtained.

In a system of the above-mentioned type, it has been found that thereceiver will lock into synchrcnism at two diiferent points, one being astable point on the vertical synchronizing impulse at which there isgood interlacing, and the other being an unstable point in front ci' thevertical impulse where there is a rise in the voltage caused by theintegration of the double frequency impulses, the latter point givingpoor interlacing and a coarse line structure.

It is the principal object of my invention to provide a televisionsystem of the above-menticned type in which the receiver will lock into.synchronism only at a point where the synchronization is stable andgives satisfactory interlaclng.

In practicing a preferred embodiment4 of -my invention, I produce asynchronizing signal of the character described above, having doublefrequency impulses preceding and followingv `each vertical synchronizingimpulse with the double frequency impulses having the same energycontent as the horizontal synchronizing impulses occurring in a liketime. Since all impulses have the same height, this means that I producethe double frequency impulses with ,a width or dural0 tion which isone-half that of the horizontal synchronizing impulses.

The invention will be better understood from the following descriptiontaken in connection with the accompanying drawings, in which 1b Figs laand 1b are curves showing the picture, blanking and synchronizingsignals which are produced in accordance with .one embodiment of myinvention at and in the` region of succeeding vertical synchronizingimpulses; I f

Figs. 2a and 2b are curves similar to those in Figs. 1aJ and 1billustrating the signals produced by another embodiment of my invention;

Fig. 3 is a diagramof a television transmitter which produces thesignals shown in Figs. la and 25 1b; f

Fig. 4 is a View of the disc illustrated in Fig. 3 for producingsynchronizing and blanking impulses;

Fig. 5 shows enlarged portions of the disc illustrated in Fig. 4; Y

Fig. 6 is a circuit diagram of a television receiver for use with thetransmitter shown in Fig. 3; and

Fig. '7 is a group of curves which are referred to in explaining myinvention.4

The invention will be described in detail by rst referring to Figures laand 1b, which illustrate the video signal which is produced andtransmitted in accordance with one embodiment of my invention. It willbeunderstood that this video signal modulates a carrier wave at thetransmitter for transmission to a receiver in the usual way. v

Referring particularly to Fig. la, the video signal. may be described asa composite signal consisting of horizontal blanking impulses l whichoccur at the end of each scanning line, picture signals 2 which aregenerated during the horizontal scanning line period, horizontalsynchronizing impulses 3 which are added to or set on top of theblanking impulses l, a blanking impulse d, of comparatively longduration as indicated by the legend, which occurs at the end of eachpicture frame or vertical deflection period,

a slotted vertical synchronizing impulse 6 which is added to or set ontop of the vertical blanking impulse 4, and impulses 'I preceding andfollowing the vertical synchronizing impulses which occur at double thefrequency of the synchronizing impulses 3 and which have one-half thewidth or duration of the impulses 3.

Since the vertical synchronizing impulse 6 is slotted, it may be lookedupon as comprising a plurality of impulses 6a. The front edge of eachimpulse Ba occurs in the same time relation as the front edges of thedouble frequency impulses 1. Thus the front edges of the impulses 6afunction to hold the horizontal deflection of the television receiver insynchronism during the occurrence of the vertical synchronizing impulsejust as in the case where horizontal synchronizing impulses are set inslots in the vertical synchronizing impulse, this latter arrangementbeing described in my above-mentioned copending application and beingillustrated in Figs. 2a and 2b, which will be described later. Thereason for this action is that in the horizontal synchronizing circuitof the receiver, it is the front or sharply rising edge of a horizontalsynchronizing impulse which provides the synchronizing voltage.

In accordance with a preferred embodiment of my invention, each doublefrequency impulse 1 is made to have one-half the width of and,therefore, one-half the energy content of each horizontal synchronizingimpulse 3. 'Ihe reason for this will be described in detail hereinafter.

It may be noted that alternate double frequency impulses 'I may beconsidered to be equalizing impulses in that they, in cooperation withimpulses 'I occurring in the same time relation as impulses 3 wherebythey may be considered as true horizontal synchronizing impulses,correct for dissymmetry of the horizontal synchronizing impulses withrespect to the vertical synchronizing impulse for successive verticalsynchronizing impulses, as described in my above-mentioned patent.V

The curve shown in Fig. 1b is a continuation of the curve shown in Fig.1a and shows the video signal as it appears during the verticaldeflection period immediately following the deflection periodrepresented in Fig. 1a. In other words, for the specic case assumed forthe purpose of illustration, where vertical synchronizing impulses occurat the rate of 60 per second, the vertical synchronizing impulse shownin Fig. 1b occurs one-sixtieth of a second later'than the verticalsynchronizing impulse shown in Fig. 1a.. In Fig. 1b, impulsescorresponding to those in Fig. la are indicated by the same referencenumerals marked with a prime.

It will be noted that, while the vertical syn-- chronizing impulses ofFigs. la and 1bl occur in the same time relation, the horizontalsynchronizing impulses in Fig.I 1b fall halfway between the horizontalsynchronizing impulses shown in Fig. la, as indicated by the broken linex, whereby interlaced scanning is produced as described in Patent2,152,234, issued March 28, 1939, to Randall C. Ballard, and assigned tothe Radio Corporation of America.

The signal illustrated in Figs. la and 1h may be produced by apparatusor circuits of various types. By Way of example, there is illustrated inFig. 3 a transmitter of the type wherein the blanking signals andsynchronizing and equalizing signals are generated by means of arotatable disc ID having properly shaped openings therein andcooperating with lightsources I I and l2 and photoelectric cells I3 andI4.

Referring to Fig. 3, I have shown merely by way of example a transmitterin which the disc Ill rotated by a motor I6 is employed for producingsynchronizing impulses of the desired shape. Near the periphery of thedisc I0 there are openings which rotate between light source II andphotocell I3 for producing the synchronizing impulses. The other row ofopenings is located between light source I2 and photocell I4 forproducing blanking impulses. The shapes and spacings of the openingsWill be described later in connection with Figs. 4 and 5.

To produce the horizontal synchronizing impulses, the source of light II, a mask II having a slit therein, and a suitable optical system form avertical slit of light on the row of synchronizing openings. As anopening rotates past this slit of light, an amount of light dependentupon the shape of the opening passes therethrough and impinges upon thephotoelectric cell I3 to produce voltage impulses of the desired shapeat the input of an amplifier I8.

Blanking impulses are produced in a similar manner by means of the lightsource I2, a slotted mask I9 and a suitable optical system which form avertical slit of light on the row of blanking openings. Thus, an amountof light dependent upon the shape of a blanking opening reaches thephotoelectric cell I4 whereby voltage impulses of the desired shape andduration are supplied to the input of an amplier 2 I.

The device for generating the picture signal is illustrated as a cathoderay transmitter tube 22 of Well-known design. It comprises a highlyevacuated envelope haw'ng therein a cathode ZI, a grid 25, a rst anode23 and a second anode 24 for producing an electron beam. The envelopealso contains a mosaic 26 of photoelectric elements which are insulatedfrom each other and from a metallic signal plate 21.

An optical image of the subject to be transmitted is projected upon themosaic 26 whereby an electrostatic image is formed thereon. Thus, whenthe mosaic is scanned by the cathode ray, picture signals are producedwhich appear across an output resistor 28 connected between the signalplate 21 and the second anode 24. These pictures are then amplified by apicture ampliiier 29 and supplied to a radio transmitter 3l.

The cathode ray may be caused to scan the mosaic 23 by means ofhorizontal and vertical deiiecting coils 32 and 33, respectively, whichare supplied with saw-tooth current from their respective deilectingcircuits 34 and 36.

The horizontal deflection and the vertical deflection of the cathode rayof the transmitter tube are made to occur in the proper time relation bymeans of the synchronizing impulses supplied from ampliiier I8 over aconductor 31 to a separatingr or lter circuit 38 which separates thehorizontal and vertical synchronizing impulses from each other.

The deecting circuits 34 and 36 may be of any suitable design, ea-chdeecting circuit including an oscillator such as a blocking oscillator.A deflecting circuit of this character is described in Toison ReissuePatent 20,338.

Blanking signals from the output of amplifier 2| are supplied over aconductor 4I to the grid 23 of the cathode ray transmitter tube forblocking the cathode ray during the horizontal return period and duringthe vertical return period.

Since the synchronizing and blanking signals are to be transmittedtogetherl with the picture signals to a receiver, the blanking signalsare supplied through an amplifier 42 to a suitable amplifier stage inthe picture amplifier 29 for removing undesired transient signals fromthe picture signals and for producing pedestals upon which thesynchronizing impulses are to be located.

The synchronizing impulses are supplied through an amplifier 43 to alater stage in the picture amplier 29 Where they are added to thepicture and blanking signals.

Referring more in detail to the impulse-generating disc, the shapes ofthe synchronizing and blanking openings and their relative locations areshown in Fig. 4 and in the fragmentary views in Fig. 5. Referring toFigs. 4 and 5, the slotted vertical synchronizing impulses b and b"(Figs. la and 1b) are produced by the diametrically opposed gro-ups ofopenings Vi and V2. The horizontal synchronizing impulses are producedby the openings BI, and, as will appear later, by alternate doublefrequency openings h.

As shown in Fig. 5, in particular, the openings h have one-half thewidth of the openings H whereby they produce the half Width impulses 1and 1 of Figs. 1a and 1b. The slit of light which is projected upon therow of synchronizing openings is indicated at 45.

The relation between the openings H producing horizontal synchronizingimpulses and the groups of openings Vi and V2 producing verticalsynchronizing impulses is such as to produce Y interlaced scanning inthe manner described and claimed in the above-mentioned Ballard patent.As taught by Ballard, the odd-line double interlacing is obtained byhaving the number of horizontal synchronizing impulse openings and thenumber of vertical synchronizing impulse openings (i. e., groups V1 andV2) such that the former number divided by the latter number equals awhole number plus one-half.

For the specific transmitter being described, the disc is'rotated at 30rotations per second, whereby 60 vertical synchronizing impulses persecond are transmitted. To prod-nce a 44-1line picture, 441 horizontalsynchronizing openings are required, these being the openings H, onehalfthe openings' h and one-half the openings or slots in the groups oi'impulses V1 and V2. On the drawing, no attempt has been made to draw thedisc openings in the proper scale to include all. 441 openings on a discof the size shown.

Referring now to the openings for producing the blanking impulses,openings 45 and 41 are opposite the groups of openings V1 and V2,respectively, and the adjacent double frequency openings h for producingthe blanking impulses 4 and 4' (Figs. la and 1h). Opposite the openingsH, there are openings 48 for producing the b-lanlring impulses l and I.The slit of light which is projected upon the row of blanking openingsis indicated at 49. y

Referring to Fig. 6, there is illustrated a television receiver which isdesigned to receive picture and synchronizing impulses which havel beentransmitted from the above-described transmitter. The receivingapparatus includes a radio receiver 5I which selects and demodulates thedesired carrier wave whereby picture signals and synchronizing impulsesof the character described above appear in the output circuits thereof.

The picture signals and synchronizing impulses are amplified inresistance coupled amplifiers 52 and 53 and impressed upon the controlgrid 54 of a cathode ray tube 56. The cathode ray tube 56 may be ofwell-known construction comprising an evacuated envelope having thereina cathode 51, the control grid 54, a rst anode 59, and a second anode6l. Suitable deflecting means, such as delecting coils 02 and B3, areprovided for deflecting the cathode ray both horizontally and verticallyto effect scanning of a fluorescent screen 64 at the end of the tube.

The horizontal deflecting coils 62 are supplied with saw-tooth currenthaving a comparatively high frequency from a horizontal deflectingcircuit indicated at E6. rlhe vertical deflecting coils t3` are suppliedwith saw-tooth current having a comparatively low frequency from avertical deflecting circuit indicated at 61. The deflecting circuits 65and 61 may be any of several wellknown designs, such circuits generallyincluding an oscillator which can be locked in step with synchronizingimpulses impressed thereon.

The number of amplier stages in the radio receiver is so selected thatthe polarity of the synchronizing impulses impressed upon the controlgrid 54' of the cathode ray tube is negative whereby the cathode ray isblocked at the end of each scanning line, this being desirable in orderto prevent a trace on the uorescent screen during the return lineperiod.

The scanning of the fluorescent screen 64 by the cathode ray ismaintained in synchronism with the scanning at the transmitter byseparating the synchronizing impulses from the picture signal andimpressing these impulses upon the deflecting circuits 66 and 61. Thecombined picture signals and synchronizing impulses are taken from apoint in the circuit, such as the output circuit of the amplifier 52,where the synchronizing impulses are of positive polarity and areimpressed upon a separating tube 68.

The separating tube 58 may be of the threeelement type having a cathodeE9, a control grid 1I` and a plate 12. The picture signals andsynchronizing impulses are fed to the grid 1I- through a grid condenser13. A grid leak resistor 14 is connected between the grid 1| and thecathode 69. A positive potential from any suitable source is applied tothe plate 12 through a plate resistor 16.

For reasons which will be explained hereinafter, only the synchronizingimpulses appear in the output circuit of the separating tube il andthese impulses are fed through a coupling condenser -11 to two filtercircuits 18 and 19. The filter circuit 18 includes a resistor 8l offairly high resistance value connected in series with a condenser 82having comparatively large capacity whereby, due to an integrationaction, substantially only the lower frequency vertical synchronizingimpulses appear across the condenser 82 as illustrated in Fig. '7. Abattery 83 or other suitable biasing source may be provided forsupplying a bias voltage to the amplifier tube through a resistor 84.l'f preferred, the battery 83 and resistor 84 may be omitted and,instead, a grid leak resistor shunted across the condenser 32. Merely byway of example, it may be noted that the following values for lter 18are satisfactory: Resistor 8i equals 2 megohms, resistor 84 equals100,000 ohms, and condenser 82 equals m. m. f.

The lter circuit 19 consists of a small capacity condenser 86 connectedin series with a resistor 31 of fairly low resistance value whereby onlythe comparatively high frequency horizontal synchronizing impulses passthrough the condenser 86 to develop voltage impulses across the resistor81 which are impressed upon the horizontal deflecting circuit B6.

Referring briefly to the operation of the separating tube E8, it will beapparent that, since the synchronizing impulses are of positive polaritywhen impressed upon the grid 1|, they will cause a ow of grid currentwhich will charge the grid condenser 13, thereby producing a negativebias on the grid. It will be apparent that the grid 1| and the cathode69 function as rectifier electrodes to keep condenser 13 charged. Thetime constant of the circuit including the grid condenser 13 and gridresistor 14 is such that the horizontal synchronizing impulses cause asubstantially steady bias to be applied to the grid 1|. The circuit ofthe separating tube is so adjusted that the biasing voltage applied tothe grid 1| is sufficient to bias the tube beyond the cut-01T point,except during the synchronizing impulses. Hence, only the synchronizingimpulses are present in the plate circuit of tube 68.

The action of my improved synchronizing signal is shown in Fig. '1,where the synchronizing signal in the region of the even verticalimpulse is indicated at 3, 3, 1, 6a and 1 (corresponding to Fig. la),and where the synchronizing signal in the region of the odd verticalimpulse is indicated at 3', 3', 1, 6a and 1 (corresponding to Fig. 1b).

The dotted line curves A and B represent the voltage which would appearacross the integrating condenser 82 (Fig. 6) for even and odd `verticalsynchronizing impulses, respectively, if the double frequency impulses 1were the same width as the preceding impulses. It will be apparent thatunder these conditions the voltage Lacross the integrating condenser 82would rise as soon as the double frequency impulses were fed into it.Because of this, the vertical delecting circuit might lock intosynchronism ahead of the steep voltage rise caused by the impulses 6a.This would cause poor interlacing and a coarse line structure, because Aand B are different in the region of the rst few double frequencyimpulses, whereas they .approach each other closely, in the desiredmanner, in the region of the vertical synchronizing impulse.

The solid line curves C and D represent the voltage which appears acrossthe integrating `condenser 82 when the double frequency impulses areone-half the width of the preceding impulses, as illustrated in Fig. 7curves C and D being for even and odd vertical synchronizing impulses,respectively.

It will be seen that curves C and D do not rise to a higher voltagelevel in the region of the double frequency impulses than the voltagelevel in the region of the impulses 3 or impulses 3', this being due tothe reduced energy content of the double frequency impulses.Consequently, the vertical deflecting circuit of the receiver will lockinto synchronism only on the steeply rising part of the curves C and Dproduced by the impulses a, where the curves are nearly identical andthe operator cannot carelessly so adjust the receiver that theinterlacing is poor.

It should be understood that my invention is not limited to the use ofdouble frequency preparatory impulses of one-half the width of the linefrequency impulses. The important feature of my invention is to make theenergy content of the preparatory signals small enough to prevent theundesired voltage rise shown by curves A and B in Fig..'1. For example,the double frequency preparatory signals may be reduced to less thanone-half the width of the line frequency impulses. Or, they may have .awidth slightly greater than one-half the width of line frequencyimpulses providing the voltage rise during the preparatory impulses iskept so low that the receivers vertical deecting circuit will not lockin on it. Also, if the preparatory signals are produced at four timesthe line frequency, the width of the preparatory signals should beapproximately one-fourth the width of the line frequency impulses orless.

As previously indicated, the synchronizing impulses may be of thecharacter indicated in Figs. 2a and 2b, if preferred, where the doublefrequency impulses are set in slots in the vertical synchronizingimpulses as described in my abovementioned copending application.

The signals illustrated in Figs. 2a, and 2b may be produced by means ofa rotatable disc having suitable openings therein or they may beproduced by means of a system like that described in Patent 2,132,655,issued Oct. 11, 1938, in the name of John P. Smith, entitled Systems forproducing electrical impulses, and assigned to the Radio Corporation ofAmerica. In either case, the double frequency impulses are narrowed toapproximately one-half the width of the line frequency impulses or lessfor ously explained.

I claim as my invention:

1. In a television system, means for transmitthe reasons previtinghorizontal synchronizing impulses which occur at a relatively highfrequency, means for transmitting framing signals which occur at acomparatively low frequency, and means for transmitting during theperiod immediately preceding each framing signal impulses which occur ata rate which is a whole number multiple of the frequency at which saidhorizontal synchronizing impulses occur and which have a width or duringwhich is substantially less than the width or duration of each of saidhorizontal synchronizing impulses.

2. In a television system employing interlaced scanning, means fortransmitting horizontal synchronizing impulses occurring at a relativelyhigh frequency, means for transmitting framing signals occurring at arelativelylow frequency, said high and low frequencies having therelation that the low frequency goes into the high frequency a wholenumber of times plus one-half, means for transmitting additionalimpulses during the period immediately preceding each of said framingsignals, which additional signals occur half way between the horizontalsynchronizing impulses whereby, in effect, there is a group of doublefrequency impulses preceding each of said framing signals, said doublefrequency impulses having a width or duration which is not greater thansubstantially one-half the width or duration of the preceding horizontalsynchronizing impulses.

3. In a television system of the type employing odd-line interlacing andwherein the horizontal and vertical synchronizing signals are separatedfrom each other at the receiver by means of lter circuits including anintegration circuit, means for transmitting horizontal synchronizingsignals which occur at a comparatively high frequency, means fortransmitting framing signals which occur at a comparatively lowfrequency and means for transmitting impulses which occur at a wholenumber multiple of said comparatively high frequency, which immediatelyprecede said framing signals and which are sufficiently short induration to make the energy content of said multiple frequency impulsesoccurring in a certain time period not substantially greater than theenergy content of the said comparatively high frequency signalsoccurring in an equal time period.

4. The invention according to claim 3 wherein said comparatively lowfrequency goes into said comparatively high frequency a whole number oftimes plus one-half to produce double interlacing and wherein each ofsaid multiple frequency impulses has a Width or duration which is notsubstantially greater than one-half the width or duration of each of thesignals occurring at said comparatively high frequency.

5. In a television system of the type employing odd-line interlacedscanning, means for transmitting horizontal synchronizing impulses whichoccur at a comparatively high frequency, means for transmitting framingsignals which occur at a comparatively low frequency, and means fortransmitting during the periods immediately preceding and immediatelyfollowing each framing signal impulses that occur at a rate which is awhole number multiple of said comparatively high frequency and whichhave awidth or duration that is substantially less than the width orduration of each of said horizontal synchronizing impulses.

6. The invention according to claim 5 wherein said comparatively lowfrequency goes into said comparatively high frequency a whole number oftimes plus one half, wherein said multiple frequency impulses occur attwice the rate at which said horizontal synchronizing impulses occur,and wherein each of said multiple frequency impulses has a width orduration which is not substantially greater than one-half the width ofeach of said horizontal synchronizing impulses. 20

ALDA V. BEDFORD.

